C. J. B.  Hitch and Millbank, J. W., ?Nitrogen Metabolism in Lichens. VII. Nitrogenase Activity and Heterocyst Frequency in Lichens with Blue-Green Phycobionts?, New Phytologist, vol. 75, pp. 239-244, 1975.

J. Walter Thomson and Brehmer, B., American Arctic Lichens I. The Macrolichens. Columbia University Press, 1984.

O. W.  Purvis, Moore, D. M., Coppins, B. J., Hawksworth, D. L., and James, P. W., The Lichen Flora of Great Britian and Ireland. London: Natural History Museum Publications, 1992.

J. W.  Thomson and Ahti, T., ?Lichens Collected on an Alaska Highway Expedition in Alaska and Canada?, The Bryologist, vol. 97, pp. 138-157, 1994.

O.  Gilbert, Lichens. London: HarperCollins, 2000, p. 288.

?Index Fungorum [taxonomy information]?, 2008.

T.  H. Lumbsch and Huhndorf, S. M., ?Outline of Ascomycota - 2007. [Classification Information]?, Myconet, vol. 13, pp. 1-58, 2007.

J. W.  Hinds and Hinds, P. L., The Macrolichens of New England, vol. 96. China: The New York Botanical Garden Press, 2007.